T-lymphocytes play a major role in the immune response by interacting with target and antigen presenting cells. T-lymphocyte-mediated killing of target cells is a multi-step process involving adhesion of cytolytic T-lymphocytes to target cells. The initiation of the immune response to most antigens involves adhesion of helper T-lymphocytes to antigen-presenting cells.
These interactions of T-lymphocytes with target and antigen-presenting cells are highly specific and depend on the recognition of an antigen on the target or antigen-presenting cell by one of the many specific antigen receptors on the surface of T-lymphocytes.
The receptor-antigen interaction of T-lymphocytes and other cells is facilitated by various T-lymphocyte surface proteins, e.g., the antigen-receptor complex CD3 and accessory molecules CD4, LFA-1, CD8 and CD2. The interaction of T-lymphocytes and other cells is also dependent on accessory molecules, such as ICAM-1, MHC class I and II and LFA-3, that are expressed on the surface of target or antigen-presenting cells and thereby play a role in the action of T-lymphocytes. One general hypothesis is that accessory molecules on the T-lymphocytes and on the target or antigen-presenting cells interact with each other to mediate intercellular adhesion. Accordingly, these accessory molecules are believed to enhance the efficiency of lymphocyte/antigen-presenting cell and lymphocyte/target cell interactions and to be important in cell adhesion-based pathologies (such as leukocyte/endothelial cell interaction leading to pathologic inflammation) and lymphocyte recirculation. Accessory molecules are also involved in activation of lymphocytes.
One important example of cell-cell interaction mediated by accessory molecules is the specific interaction between CD2 (a T-lymphocyte accessory molecule) and LFA-3 (a target cell accessory molecule). CD2/LFA-3 binding appears to be essential for many important cell-cell reactions, including the initiation of the T-lymphocyte functional responses (Dustin et al., J. Exp. Med., 165, pp. 677-92 (1987); Springer et al., Ann. Rev. Immunol., 5, pp. 223-52 (1987)). The importance of the CD2/LFA-3 complex in cell-cell adhesion is indicated by the findings that purified LFA-3 binds to CD2 on the surface of T-lymphocytes (Dustin et al., J. Exp. Med., 165, pp. 677-92 (1987)), that CD2 purified from T-lymphocytes binds LFA-3 on cell surfaces and inhibits the binding of LFA-3-specific monoclonal antibodies ("MAbs") to LFA-3 (Selvaraj et al., Nature, 326, pp. 400-403 (1987)), and that rosetting of human erythrocytes, which express LFA-3, to cells expressing CD2 is blocked by anti-LFA-3 MAbs and anti-CD2 MAbs (see, e.g., Seed et al., Proc. Natl. Acad. Sci. USA, 84, pp. 3365-69 (1987)).
LFA-3, which is found on the surface of a wide variety of cells including monocytes, granulocytes, T-lymphocytes, erythrocytes, B-lymphoblastoid cell lines, thymic epithelial cells, and vascular endothelial cells, has become the subject of a considerable amount of study to further elucidate its role in various T-lymphocyte interactions. Two natural forms of LFA-3 have been identified. One form of LFA-3 ("transmembrane LFA-3") is anchored in the cell membrane by a transmembrane hydrophobic domain. cDNA encoding this form of LFA-3 has been cloned and sequenced (see, e.g., Wallner et al., J. Exp. Med., 166, pp. 923-32 (1987)). Another form of LFA-3 is anchored to the cell membrane via a covalent linkage to phosphatidylinositol ("PI") -containing glycolipid. This latter form has been designated "PI-linked LFA-3", and cDNA encoding this form of LFA-3 has also been cloned and sequenced (Wallner et al., PCT patent application WO 90/02181).
Although the DNA sequence of the LFA-3 gene and the primary amino acid sequences of both forms of LFA-3 have been determined, the actual site of interaction between LFA-3 and its receptor, CD2, has not previously been identified. There is a need to identify the CD2-binding domain on LFA-3 in order to better understand the specific interaction between CD2 and LFA-3 and, thereby, to effect and modulate the cellular and immunological processes that are dependent on the formation of the CD2/LFA-3 complex. Such information would also be useful in a variety of other applications including diagnostic and therapeutic compositions, protein purification, antibody identification and purification, and comparative and structural studies of LFA-3 and other proteins.